Arch manual pages

A daemon is a service process that runs in the background and supervises the
system or provides functionality to other processes. Traditionally, daemons
are implemented following a scheme originating in SysV Unix. Modern daemons
should follow a simpler yet more powerful scheme (here called
"new-style" daemons), as implemented by systemd(1). This
manual page covers both schemes, and in particular includes recommendations
for daemons that shall be included in the systemd init system.

When a traditional SysV daemon starts, it should execute the following steps as
part of the initialization. Note that these steps are unnecessary for
new-style daemons (see below), and should only be implemented if compatibility
with SysV is essential.

1.Close all open file descriptors except standard input,
output, and error (i.e. the first three file descriptors 0, 1, 2). This
ensures that no accidentally passed file descriptor stays around in the daemon
process. On Linux, this is best implemented by iterating through
/proc/self/fd, with a fallback of iterating from file descriptor 3 to the
value returned by getrlimit() for RLIMIT_NOFILE.

2.Reset all signal handlers to their default. This is
best done by iterating through the available signals up to the limit of
_NSIG and resetting them to SIG_DFL.

10.In the daemon process, reset the umask to 0, so that
the file modes passed to open(), mkdir() and suchlike directly
control the access mode of the created files and directories.

11.In the daemon process, change the current directory to
the root directory (/), in order to avoid that the daemon involuntarily blocks
mount points from being unmounted.

12.In the daemon process, write the daemon PID (as
returned by getpid()) to a PID file, for example /run/foobar.pid (for a
hypothetical daemon "foobar") to ensure that the daemon cannot be
started more than once. This must be implemented in race-free fashion so that
the PID file is only updated when it is verified at the same time that the PID
previously stored in the PID file no longer exists or belongs to a foreign
process.

13.In the daemon process, drop privileges, if possible
and applicable.

14.From the daemon process, notify the original process
started that initialization is complete. This can be implemented via an
unnamed pipe or similar communication channel that is created before the first
fork() and hence available in both the original and the daemon
process.

15.Call exit() in the original process. The
process that invoked the daemon must be able to rely on that this
exit() happens after initialization is complete and all external
communication channels are established and accessible.

The BSD daemon() function should not be used, as it
implements only a subset of these steps.

A daemon that needs to provide compatibility with SysV systems
should implement the scheme pointed out above. However, it is recommended to
make this behavior optional and configurable via a command line argument to
ease debugging as well as to simplify integration into systems using
systemd.

Modern services for Linux should be implemented as new-style daemons. This makes
it easier to supervise and control them at runtime and simplifies their
implementation.

For developing a new-style daemon, none of the initialization
steps recommended for SysV daemons need to be implemented. New-style init
systems such as systemd make all of them redundant. Moreover, since some of
these steps interfere with process monitoring, file descriptor passing and
other functionality of the init system, it is recommended not to execute
them when run as new-style service.

Note that new-style init systems guarantee execution of daemon
processes in a clean process context: it is guaranteed that the environment
block is sanitized, that the signal handlers and mask is reset and that no
left-over file descriptors are passed. Daemons will be executed in their own
session, with standard input connected to /dev/null and standard
output/error connected to the systemd-journald.service(8) logging
service, unless otherwise configured. The umask is reset.

It is recommended for new-style daemons to implement the
following:

1.If SIGTERM is received, shut down the daemon
and exit cleanly.

2.If SIGHUP is received, reload the configuration
files, if this applies.

3.Provide a correct exit code from the main daemon
process, as this is used by the init system to detect service errors and
problems. It is recommended to follow the exit code scheme as defined in the
LSB recommendations for SysV init scripts[1].

4.If possible and applicable, expose the daemon's
control interface via the D-Bus IPC system and grab a bus name as last step of
initialization.

5.For integration in systemd, provide a .service unit
file that carries information about starting, stopping and otherwise
maintaining the daemon. See systemd.service(5) for details.

6.As much as possible, rely on the init system's
functionality to limit the access of the daemon to files, services and other
resources, i.e. in the case of systemd, rely on systemd's resource limit
control instead of implementing your own, rely on systemd's privilege dropping
code instead of implementing it in the daemon, and similar. See
systemd.exec(5) for the available controls.

7.If D-Bus is used, make your daemon bus-activatable by
supplying a D-Bus service activation configuration file. This has multiple
advantages: your daemon may be started lazily on-demand; it may be started in
parallel to other daemons requiring it — which maximizes
parallelization and boot-up speed; your daemon can be restarted on failure
without losing any bus requests, as the bus queues requests for activatable
services. See below for details.

8.If your daemon provides services to other local
processes or remote clients via a socket, it should be made socket-activatable
following the scheme pointed out below. Like D-Bus activation, this enables
on-demand starting of services as well as it allows improved parallelization
of service start-up. Also, for state-less protocols (such as syslog, DNS), a
daemon implementing socket-based activation can be restarted without losing a
single request. See below for details.

9.If applicable, a daemon should notify the init system
about startup completion or status updates via the sd_notify(3)
interface.

10.Instead of using the syslog() call to log
directly to the system syslog service, a new-style daemon may choose to simply
log to standard error via fprintf(), which is then forwarded to syslog
by the init system. If log levels are necessary, these can be encoded by
prefixing individual log lines with strings like "<4>" (for
log level 4 "WARNING" in the syslog priority scheme), following a
similar style as the Linux kernel's printk() level system. For details,
see sd-daemon(3) and systemd.exec(5).

These recommendations are similar but not identical to the
Apple MacOS X Daemon Requirements[2].

New-style init systems provide multiple additional mechanisms to activate
services, as detailed below. It is common that services are configured to be
activated via more than one mechanism at the same time. An example for
systemd: bluetoothd.service might get activated either when Bluetooth hardware
is plugged in, or when an application accesses its programming interfaces via
D-Bus. Or, a print server daemon might get activated when traffic arrives at
an IPP port, or when a printer is plugged in, or when a file is queued in the
printer spool directory. Even for services that are intended to be started on
system bootup unconditionally, it is a good idea to implement some of the
various activation schemes outlined below, in order to maximize
parallelization. If a daemon implements a D-Bus service or listening socket,
implementing the full bus and socket activation scheme allows starting of the
daemon with its clients in parallel (which speeds up boot-up), since all its
communication channels are established already, and no request is lost because
client requests will be queued by the bus system (in case of D-Bus) or the
kernel (in case of sockets) until the activation is completed.

Old-style daemons are usually activated exclusively on boot (and manually by the
administrator) via SysV init scripts, as detailed in the LSB Linux Standard
Base Core Specification[1]. This method of activation is supported
ubiquitously on Linux init systems, both old-style and new-style systems.
Among other issues, SysV init scripts have the disadvantage of involving shell
scripts in the boot process. New-style init systems generally employ updated
versions of activation, both during boot-up and during runtime and using more
minimal service description files.

In systemd, if the developer or administrator wants to make sure
that a service or other unit is activated automatically on boot, it is
recommended to place a symlink to the unit file in the .wants/ directory of
either multi-user.target or graphical.target, which are normally used as
boot targets at system startup. See systemd.unit(5) for details about
the .wants/ directories, and systemd.special(7) for details about the
two boot targets.

In order to maximize the possible parallelization and robustness and simplify
configuration and development, it is recommended for all new-style daemons
that communicate via listening sockets to employ socket-based activation. In a
socket-based activation scheme, the creation and binding of the listening
socket as primary communication channel of daemons to local (and sometimes
remote) clients is moved out of the daemon code and into the init system.
Based on per-daemon configuration, the init system installs the sockets and
then hands them off to the spawned process as soon as the respective daemon is
to be started. Optionally, activation of the service can be delayed until the
first inbound traffic arrives at the socket to implement on-demand activation
of daemons. However, the primary advantage of this scheme is that all
providers and all consumers of the sockets can be started in parallel as soon
as all sockets are established. In addition to that, daemons can be restarted
with losing only a minimal number of client transactions, or even any client
request at all (the latter is particularly true for state-less protocols, such
as DNS or syslog), because the socket stays bound and accessible during the
restart, and all requests are queued while the daemon cannot process them.

New-style daemons which support socket activation must be able to
receive their sockets from the init system instead of creating and binding
them themselves. For details about the programming interfaces for this
scheme provided by systemd, see sd_listen_fds(3) and
sd-daemon(3). For details about porting existing daemons to
socket-based activation, see below. With minimal effort, it is possible to
implement socket-based activation in addition to traditional internal socket
creation in the same codebase in order to support both new-style and
old-style init systems from the same daemon binary.

systemd implements socket-based activation via .socket units,
which are described in systemd.socket(5). When configuring socket
units for socket-based activation, it is essential that all listening
sockets are pulled in by the special target unit sockets.target. It is
recommended to place a WantedBy=sockets.target directive in the
"[Install]" section to automatically add such a dependency on
installation of a socket unit. Unless DefaultDependencies=no is set,
the necessary ordering dependencies are implicitly created for all socket
units. For more information about sockets.target, see
systemd.special(7). It is not necessary or recommended to place any
additional dependencies on socket units (for example from multi-user.target
or suchlike) when one is installed in sockets.target.

When the D-Bus IPC system is used for communication with clients, new-style
daemons should employ bus activation so that they are automatically activated
when a client application accesses their IPC interfaces. This is configured in
D-Bus service files (not to be confused with systemd service unit files!). To
ensure that D-Bus uses systemd to start-up and maintain the daemon, use the
SystemdService= directive in these service files to configure the
matching systemd service for a D-Bus service. e.g.: For a D-Bus service whose
D-Bus activation file is named org.freedesktop.RealtimeKit.service, make sure
to set SystemdService=rtkit-daemon.service in that file to bind it to
the systemd service rtkit-daemon.service. This is needed to make sure that the
daemon is started in a race-free fashion when activated via multiple
mechanisms simultaneously.

Often, daemons that manage a particular type of hardware should be activated
only when the hardware of the respective kind is plugged in or otherwise
becomes available. In a new-style init system, it is possible to bind
activation to hardware plug/unplug events. In systemd, kernel devices
appearing in the sysfs/udev device tree can be exposed as units if they are
tagged with the string "systemd". Like any other kind of unit, they
may then pull in other units when activated (i.e. plugged in) and thus
implement device-based activation. systemd dependencies may be encoded in the
udev database via the SYSTEMD_WANTS= property. See
systemd.device(5) for details. Often, it is nicer to pull in services
from devices only indirectly via dedicated targets. Example: Instead of
pulling in bluetoothd.service from all the various bluetooth dongles and other
hardware available, pull in bluetooth.target from them and bluetoothd.service
from that target. This provides for nicer abstraction and gives administrators
the option to enable bluetoothd.service via controlling a
bluetooth.target.wants/ symlink uniformly with a command like enable of
systemctl(1) instead of manipulating the udev ruleset.

Often, runtime of daemons processing spool files or directories (such as a
printing system) can be delayed until these file system objects change state,
or become non-empty. New-style init systems provide a way to bind service
activation to file system changes. systemd implements this scheme via
path-based activation configured in .path units, as outlined in
systemd.path(5).

Some daemons that implement clean-up jobs that are intended to be executed in
regular intervals benefit from timer-based activation. In systemd, this is
implemented via .timer units, as described in systemd.timer(5).

Other forms of activation have been suggested and implemented in some systems.
However, there are often simpler or better alternatives, or they can be put
together of combinations of the schemes above. Example: Sometimes, it appears
useful to start daemons or .socket units when a specific IP address is
configured on a network interface, because network sockets shall be bound to
the address. However, an alternative to implement this is by utilizing the
Linux IP_FREEBIND socket option, as accessible via FreeBind=yes
in systemd socket files (see systemd.socket(5) for details). This
option, when enabled, allows sockets to be bound to a non-local, not
configured IP address, and hence allows bindings to a particular IP address
before it actually becomes available, making such an explicit dependency to
the configured address redundant. Another often suggested trigger for service
activation is low system load. However, here too, a more convincing approach
might be to make proper use of features of the operating system, in
particular, the CPU or I/O scheduler of Linux. Instead of scheduling jobs from
userspace based on monitoring the OS scheduler, it is advisable to leave the
scheduling of processes to the OS scheduler itself. systemd provides
fine-grained access to the CPU and I/O schedulers. If a process executed by
the init system shall not negatively impact the amount of CPU or I/O bandwidth
available to other processes, it should be configured with
CPUSchedulingPolicy=idle and/or IOSchedulingClass=idle.
Optionally, this may be combined with timer-based activation to schedule
background jobs during runtime and with minimal impact on the system, and
remove it from the boot phase itself.

When writing systemd unit files, it is recommended to consider the following
suggestions:

1.If possible, do not use the Type=forking
setting in service files. But if you do, make sure to set the PID file path
using PIDFile=. See systemd.service(5) for details.

2.If your daemon registers a D-Bus name on the bus, make
sure to use Type=dbus in the service file if possible.

3.Make sure to set a good human-readable description
string with Description=.

4.Do not disable DefaultDependencies=, unless you
really know what you do and your unit is involved in early boot or late system
shutdown.

5.Normally, little if any dependencies should need to be
defined explicitly. However, if you do configure explicit dependencies, only
refer to unit names listed on systemd.special(7) or names introduced by
your own package to keep the unit file operating system-independent.

6.Make sure to include an "[Install]" section
including installation information for the unit file. See
systemd.unit(5) for details. To activate your service on boot, make
sure to add a WantedBy=multi-user.target or
WantedBy=graphical.target directive. To activate your socket on boot,
make sure to add WantedBy=sockets.target. Usually, you also want to
make sure that when your service is installed, your socket is installed too,
hence add Also=foo.socket in your service file foo.service, for a
hypothetical program foo.

At the build installation time (e.g. make install during package build),
packages are recommended to install their systemd unit files in the directory
returned by pkg-config systemd --variable=systemdsystemunitdir (for
system services) or pkg-config systemd --variable=systemduserunitdir
(for user services). This will make the services available in the system on
explicit request but not activate them automatically during boot. Optionally,
during package installation (e.g. rpm -i by the administrator),
symlinks should be created in the systemd configuration directories via the
enable command of the systemctl(1) tool to activate them
automatically on boot.

Packages using autoconf(1) are recommended to use a
configure script excerpt like the following to determine the unit
installation path during source configuration:

This snippet allows automatic installation of the unit files on
systemd machines, and optionally allows their installation even on machines
lacking systemd. (Modification of this snippet for the user unit directory
is left as an exercise for the reader.)

Additionally, to ensure that make distcheck continues to
work, it is recommended to add the following to the top-level Makefile.am
file in automake(1)-based projects:

In the rpm(8) .spec file, use snippets like the following
to enable/disable the service during installation/deinstallation. This makes
use of the RPM macros shipped along systemd. Consult the packaging
guidelines of your distribution for details and the equivalent for other
package managers.

If the service shall be restarted during upgrades, replace the
"%postun" scriptlet above with the following:

%postun
%systemd_postun_with_restart foobar.service

Note that "%systemd_post" and "%systemd_preun"
expect the names of all units that are installed/removed as arguments,
separated by spaces. "%systemd_postun" expects no arguments.
"%systemd_postun_with_restart" expects the units to restart as
arguments.

To facilitate upgrades from a package version that shipped only
SysV init scripts to a package version that ships both a SysV init script
and a native systemd service file, use a fragment like the following:

Where 0.47.11-1 is the first package version that includes the
native unit file. This fragment will ensure that the first time the unit
file is installed, it will be enabled if and only if the SysV init script is
enabled, thus making sure that the enable status is not changed. Note that
chkconfig is a command specific to Fedora which can be used to check
whether a SysV init script is enabled. Other operating systems will have to
use different commands here.

Since new-style init systems such as systemd are compatible with traditional
SysV init systems, it is not strictly necessary to port existing daemons to
the new style. However, doing so offers additional functionality to the
daemons as well as simplifying integration into new-style init systems.

To port an existing SysV compatible daemon, the following steps
are recommended:

1.If not already implemented, add an optional command
line switch to the daemon to disable daemonization. This is useful not only
for using the daemon in new-style init systems, but also to ease
debugging.

2.If the daemon offers interfaces to other software
running on the local system via local AF_UNIX sockets, consider
implementing socket-based activation (see above). Usually, a minimal patch is
sufficient to implement this: Extend the socket creation in the daemon code so
that sd_listen_fds(3) is checked for already passed sockets first. If
sockets are passed (i.e. when sd_listen_fds() returns a positive
value), skip the socket creation step and use the passed sockets. Secondly,
ensure that the file system socket nodes for local AF_UNIX sockets used
in the socket-based activation are not removed when the daemon shuts down, if
sockets have been passed. Third, if the daemon normally closes all remaining
open file descriptors as part of its initialization, the sockets passed from
the init system must be spared. Since new-style init systems guarantee that no
left-over file descriptors are passed to executed processes, it might be a
good choice to simply skip the closing of all remaining open file descriptors
if sockets are passed.

3.Write and install a systemd unit file for the service
(and the sockets if socket-based activation is used, as well as a path unit
file, if the daemon processes a spool directory), see above for details.

4.If the daemon exposes interfaces via D-Bus, write and
install a D-Bus activation file for the service, see above for details.